The evaluation of high temperature superconductors is becoming an important issue for those involved in research and fabrication of these materials. Currently, in spite of great expectations, understanding of basic physics and commercial application of these materials are in very initial stages.
The conventional ways of characterizing high-T.sub.c superconducting materials are to measure their basic properties: critical temperature T.sub.c, critical current, heat capacity jump, structural uniformity and imperfections such as impurities, grain and twin boundaries. The jump in heat capacity at the critical temperature is measured in calorimetric experiments and addresses a basic feature of a superconducting transition as a second order phase transition. Critical temperature and critical current are usually determined from temperature dependence of electrical DC or microwave resistance [M. Tinkham, Introduction to Superconductivity, McGraw-Hill, N.Y., 1975]. Structural uniformity and imperfections are monitored by X-ray and optical microscopy techniques. Measurements of DC and microwave resistance in zero and non-zero magnetic field are most frequently used because they directly address the manifestations of superconductivity. However, these methods are much less effective for the high-T.sub.c superconducting materials, than for conventional superconductors (metals). The high-T.sub.c superconducting materials are, by nature, very anisotropic and difficult to grow in large volumes. For successful implementation of the traditional methods, (both electrical and thermal), samples of rather large volumes(.about.1 mm.sup.3) and high level of uniformity are required. The current technology is unable to provide uniform single crystals of a size larger than .about.1 mm. None of the techniques disclosed above has allowed to evaluate characteristics of small volumes (sample size &lt;100 .mu.m) of high-T.sub.c superconducting materials with required accuracy and sensitivity to the anisotropy and structural defects.
The next approach to the problem has been initiated by development of a photothermal technique which was earlier employed for evaluation of thermal properties of semiconductors, metals and insulating materials. The teaching of the U.S. Pat. Nos. 4,579,463; 4,634,290; 4,795,260; 5,074,669 and 5,228,776 is directed to photothermal measurements of semiconductor wafers. In the techniques disclosed, the surface of the sample is irradiated by light beams of a pump laser and a probe laser, both focused onto the sample surface and separated by a predetermined distance. The intensity of the pump laser beam is periodically modulated which causes heating of a spot on the surface of the sample on which the light beam is focused. The temperature of the spot varies synchronously with the modulation of the pump laser source light beam. The thermal wave excited by the pump source beam propagates along the sample and modulates its dielectric constant. To measure reflectivity, a second probing optical beam is used. The measured AC intensity of the reflected probe beam and the phase delay between the modulation of the incident beam and that of the reflected probe beam are used to obtain the thermal diffusivity of the material. The existing teaching of the photothermal technique neglects the anisotropic optical properties of high-T.sub.c materials and their nonuniformity causing problems for measurements over wide temperature range. In the existing form, the photothermal technique is not suitable for measuring of temperature dependence of thermal and optical parameters of high-T.sub.c materials.
For characterization of high-T.sub.c materials, the existing photothermal technique has to be substantially modified. Strong anisotropy of these materials makes the direction of polarization of the probe beam an important characterization tool. Nonuniformity of the high-T.sub.c samples demands special equipment to control the relative position of samples and the optical means during the measurement cycle, namely to change the direction of polarization of the probe beam relative to the high-T.sub.c sample.